Increase window size to 64kB. Line rate downloads on a 100Mbps link,

anyone?

1 files changed, 30 insertions, 9 deletions

diff --git a/src/include/gpxe/tcp.h b/src/include/gpxe/tcp.h
index c8988cf52..494e2ab9a 100644
--- a/src/include/gpxe/tcp.h
+++ b/src/include/gpxe/tcp.h
@@ -213,18 +213,39 @@ struct tcp_mss_option {
 /**
  * Advertised TCP window size
  *
+ * 
  * Our TCP window is actually limited by the amount of space available
  * for RX packets in the NIC's RX ring; we tend to populate the rings
- * with far fewer descriptors than a typical driver.  Since we have no
- * way of knowing how much of this RX ring space will be available for
- * received TCP packets (consider, for example, that they may all be
- * consumed by a series of unrelated ARP requests between other
- * machines on the network), it is actually not even theoretically
- * possible for us to specify an accurate window size.  We therefore
- * guess an arbitrary number that is empirically as large as possible
- * while avoiding retransmissions due to dropped packets.
+ * with far fewer descriptors than a typical driver.  This would
+ * result in a desperately small window size, which kills WAN download
+ * performance; the maximum bandwidth on any link is limited to
+ *
+ *    max_bandwidth = ( tcp_window / round_trip_time )
+ *
+ * With a 4kB window, which probably accurately reflects our amount of
+ * buffer space, and a WAN RTT of say 200ms, this gives a maximum
+ * achievable bandwidth of 20kB/s, which is not acceptable.
+ *
+ * We therefore aim to process packets as fast as they arrive, and
+ * advertise an "infinite" window.  If we don't process packets as
+ * fast as they arrive, then we will drop packets and have to incur
+ * the retransmission penalty.
+ *
+ * Since we don't store out-of-order received packets, the
+ * retransmission penalty is that the whole window contents must be
+ * resent.
+ *
+ * We choose to compromise on a window size of 64kB (which is the
+ * maximum that can be represented without using TCP options).  This
+ * gives a maximum bandwidth of 320kB/s at 200ms RTT, which is
+ * probably faster than the actual link bandwidth.  It also limits
+ * retransmissions to 64kB, which is reasonable.
+ *
+ * Finally, since the window goes into a 16-bit field and we cannot
+ * actually use 65536, we use a window size of (65536-4) to ensure
+ * that payloads remain dword-aligned.
  */
-#define TCP_WINDOW_SIZE	4096
+#define TCP_WINDOW_SIZE	( 65536 - 4 )
 
 /**
  * Advertised TCP MSS